1. Field of the Invention
The present invention relates to an idling engine speed control apparatus of a vehicle, and particularly to an idling engine speed control apparatus which can economically and suitably control the idling engine speed in the case where a variable displacement type compressor with a variable displacement is mounted as a part of an automotive air conditioning system.
2. Description of the Related Art
An idling engine speed of a vehicle is changed by load change of an engine. For example, a variable displacement type compressor mounted for an automotive air conditioner is driven, its driving force is applied as a load to the engine, so that the idling engine speed is lowered.
Particularly, in the variable displacement type compressor, the displacement of a compression refrigerant gas is changed, and in the case where the compressor is driven in the state where the displacement is large, the load of the engine becomes high, and the idling engine speed is excessively lowered, which results in an engine stop.
In order to avoid such a disadvantage, in the period when the variable displacement type compressor is driven, idling up is made on the assumption that the displacement is maximum, and the idling engine speed is maintained. However, in such idling up, since the standard is the case where the displacement becomes maximum as described above, when the displacement is decreased and the engine load becomes low, the engine speed becomes high more than needs and the fuel efficiency becomes poor.
Then, in the case where this kind of variable displacement type compressor is used as a part of an automotive air conditioning system, in order to perform idling up in accordance with the increase or decrease of the displacement, the displacement is calculated according to, for example, a method (1) or (2) described below, and the output of the engine is adjusted on the basis of this calculation result to stabilize the idling engine speed.
(1) Attention is paid to the fact that there is a definite correlation between the displacement of the variable displacement type compressor and a position of a control mechanism part for controlling the displacement, and the position of the control mechanism part is detected so that the displacement is calculated.
(2) Attention is paid to the fact that there a definite correlation between a discharge pressure and the displacement, and the discharge pressure is detected so that the displacement is calculated.
According to the foregoing conventional control of idling engine speed, idling up in accordance with the increase or decrease of the displacement becomes possible from calculation of the displacement of the variable displacement type compressor and from adjustment of an engine output on the basis of the calculation result. However, since the displacement is calculated on the basis of the position of the control mechanism part or discharge pressure, a position detecting sensor of the control mechanism part or a discharge pressure detecting sensor is indispensable, so that the control cost becomes high by that, and the cost of the vehicle itself also becomes high. Besides, there is also a problem that when a poor operation occurs in this kind of sensor, accurate displacement can not be calculated, and it is impossible to perform suitable idling up in accordance with an engine load corresponding to the present displacement.
The present invention has been made in view of the above, and an object thereof is to provide an idling engine speed control apparatus which can economically and suitably control an idling engine speed in the case where a variable displacement type compressor with a variable displacement is mounted as a part of an automotive air conditioning system.
In order to achieve the foregoing object, according to a first aspect of the invention, in an idling engine speed control apparatus of a vehicle including a variable displacement type compressor in which a displacement is determined by a differential pressure between a control pressure and a suction pressure, an engine output at idling of the vehicle is controlled on the basis of an external signal for controlling the differential pressure, sent to a control valve for controlling the differential pressure.
According to a second aspect of the invention, the control pressure is controlled by adjustment of the control valve, the external signal for controlling the differential pressure is formed of a duty ratio of a control valve current value sent to the control valve, and the engine output at idling is controlled on the basis of the duty ratio.
According to the invention, the engine output at idling is controlled on the basis of the external signal for controlling the differential pressure of the variable displacement type compressor, and the idling engine speed is adjusted.